Programmable function generator

ABSTRACT

A programmable function generator, which is suitable for construction in integrated circuit form, includes a current ladder which produces an output voltage or current which is a programmable function of an input voltage or current at n evenly spaced values of the input voltage or current corresponding to n spaced output points along the current ladder. For values of the input signal between any two of the n values, the output is a linear interpolation between the output values at the corresponding output points along the current ladder. The programming of each point is independent of all other points, and the output at any point is established by the ratio of a resistor pair. Accurately controlled current sources or current drivers control the flow of currents to each of the n points and the flow of current from terminals of said current ladder including compensated current drivers, one of which differentially controls current flow from end terminals of the current ladder in response to a variable input control voltage or current which, for example may be a linear ramp of voltage or current.

United States Patent 191 Pace [451 June 19, 1973 1 PROGRAMMABLE FUNCTIONGENERATOR [75] Inventor: JohnW.Pace,Aloha,Oreg.

[73] Assignee: Telttronix, 1nc., Tektronix lndustrial Park. Beaverton.Oreg.

I22] Filed: Feb. 28, 1972 [2|] App]. No.: 229,901

Primary ExaminerJoseph F. Ruggiero Attorney-Stephen W. Blore, Kenneth S.Klarquist,

Joseph B Sparkmanet al.

[57] ABSTRACT A programmable function generator, which is suitable forconstruction in integrated circuit form, includes a current ladder whichproduces an output voltage or current which is a programmable functionof an input voltage or current at n evenly spaced values of the inputvoltage or current corresponding to n spaced output points along thecurrent ladder. For values of the input signal between any two of the nvalues, the output is a linear interpolation between the output valuesat the corresponding output points along the current ladder. Theprogramming of each point is independent of all other points, and theoutput at any point is estab lished by the ratio of a resistor pair.Accurately controlled current sources or current drivers control theflow of currents to each of the n points and the flow of current fromterminals of said current ladder including compensated current drivers,one of which differentially controls current flow from end terminals ofthe current ladder in response to a variable input control voltage orcurrent which, for example may be a linear ramp of voltage or current.

14 Claims, 4 Drawing Figures Patented June 19, 1973 2 Sheets-Sheet 1PROGRAMMABLE FUNCTION GENERATOR BACKGROUND OF THE INVENTION Thisinvention relates to the field of function generators for constructingor simulating electrical waveforms, for example a sine wave as well asasymmetrical or other variant waveforms, in response to an input signal.Many function generators can also be programmed to produce asymmetricalor other variant waveform outputs. Prior art examples of thisprogrammable variety includes diode type function generators, servomotor systems using cams and drums, and digital memory systems.

The diode type generators are often difficult to program and do not haveindependent adjustments at each programming point. Furthermore, whilethey are capable of responding to input signals relatively fast, theyare rather sensitive to temperature variation. Servo motor systems areusually large and expensive consuming a considerable amount of powerwhile responding slowly to the input waveform. Digital systems also havenumerous drawbacks. They are usually expensive because the inputwaveform must be converted from analog to digital form, operated on andthen reconverted to analog form. Thus, they require analog to digitaland digital to analog converters as well as a digital memory unit.Furthermore, this mode of operation results in an output which varies indiscrete steps.

SUMMARY OF THE INVENTION Programmable function generators, according tothe present invention, have the advantages of being small andinexpensive to make, and of consuming little power and respondingrapidly to relatively fast changing input signals. It is contemplatedthat almost the entire system can be put into one or more integratedcircuits.

The programmable function generator includes a current ladder forsequentially actuating a plurality of output transistors. When actuated,each transistor connects a particular resistor pair to a current source.A selected portion of this current flow is utilized to produce theoutput waveform. The programming is accomplished by varying the ratiosof each resistor pair, each of which may be programmed independent ofthe others.

The current ladder is supplied with equal currents delivered to nodepoints along the ladder and is operated by a differential current driverwhich causes smoothly changing complementary currents to flow from endterminals of the ladder in response to an external signal. As a functionof this current flow each of the various node points of the currentladder will be sequentially at a voltage .peak sufficient to actuate itsassociated output transistor. A voltage limiting circuit enables thelength of the current ladder to be increased to thereby increase thenumber of programmable points which can be employed without producingexcessive voltage drops along the ladder, and compensated currentsources or current drivers stabilize the operation of the functiongenerator against temperature variations of the properties of thecomponents of the circuit.

Since each point along the ladder is independently programmable, thefunction generator of the present invention lends itself to computerprogramming and other means of precisely and rapidly varying the outputwaveform. Furthermore, if precision potentiometers are employed as theresistor pairs, accurate simulation of nonlinear functions can beobtained.

It is therefore an object of the present invention to provide animproved function generatorcircuit having an independently programmableoutput.

It is a further object of the present invention to provide an improvedtransistorized function generator circuit adapted for integrated circuitfabrication.

It is another object of the present invention to provide a functiongenerator whose output will not change due to variation in componentcharacteristics due to temperature variation.

It is a further object of the present invention to provide means formaking the output waveform between the programmable points a linearinterpolation of the output at said points.

It is a still further object of the present invention to linearize thecomplementary shifting of current flowing to the current driver from theend terminals of the current ladder.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of afunction generator according to the present invention;

FIG. 2 is a time sequence table illustrating the various relationshipsamong the input voltage, the currents from the current ladder'and theoccurrence of voltage peaks at the node points of the ladder;

FIG. 3 is a schematic diagram of a compensated current source accordingto the present invention; and

FIG. 4 is a schematic diagram of a differential current driver accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The circuit shown inFIG. 1 includes a current ladder 10, an output resistor network 11, acurrent regulator 12, a differential current driver 14 and a voltagelimiter 16.

The current ladder 10 has a pair of terminals 20, 22 disposed at each ofits ends. The current ladder includes a plurality of reversely connectedparallel diode pairs including the pairs 24,26; 28,30; 32,34 and 36,38disposed in series between the input terminals. The reversely connecteddiodes separate and define node points including node points 40, 42, 44,46 and 48; the nodes 40 and 48 being at the terminals 20 and 22,respectively. Current sources 50, 52, 54, 56 and 58 each provide a unitcurrent I from a positive voltage supply 59 to nodes 40, 42, 44, 46 and48, respectively. Also connected to these nodes are the bases andcollectors of diode connected transistors 60, 62, 64, 66 and 68,respectively, the emitters of which are connected to a common terminal70. These diode connected transistors function as, and may be replacedby, conventional diodes. The nodes are also connected to the bases oftransistors 72, 74, 76, 78 and 80, respectively. The emitters of thesetransistors are all connected to a negative voltage supply 82 through aterminal 83 and a current source 84. When conducting, the emitter ofeach transistor supplies a current to source 84 from the output resistornetwork 11. While five stages or nodes are shown, it will be understoodthat a considerably larger number of such stages with associated nodepoints and connected diodes and transistors will ordinarily be employed.

Terminal 70 is connected to the negative input of a differential voltageamplifier 85 forming part of the current regulator circuit 12 which alsoincludes matched resistors 86, 88 and another unit current source 90.The resistor 86 is connected between the output of the current source 90and the positive terminal of a voltage source 91. The resistor 88 isconnected between the terminal 70 and the same positive voltage sourceterminal. The voltage drop developed across resistor 88 from terminal 70to the voltage source 91 is thus applied to the negative input of thedifferential voltage amplifier 85, while the voltage drop acrossresistor 86 from current source'90 to the voltage source 91 is appliedto the positive input of the amplifier 85. The output of thedifferential voltage amplifier 85 is fed back to the unit currentsources 50, 52, 54, 56, 58 and 90 to regulate the current flow throughsuch sources to maintain the current through terminal 70 at a precisevalue substantially equal to each of the unit currents I.

If the current through terminal 70 should decrease relative to thecurrent from source 90, the voltage drop across resistor 88 willdecrease correspondingly to cause the voltage at the negative input tothe differential voltage amplifier 85 to decrease. A decrease in thevoltage at the negative input results in an increased positive outputwhich causes an increase in current flow from the unit current sources.In a like manner, if

the current flow through terminal 70 increases relative to the currentflow from unit'current source 90, the.

voltage output from the amplifier 85 to the current sources willdecrease until the current flow through terminal .70 returns to itsproper unit value.

For purposes of explaining the operation of the current ladder 10, itwill first be assumed that all current flowing from the current ladderexcept that flowing through the terminal'70 flows through the terminals20 and 22, the sum of the currents flowing through the terminals 20 and22- being designated i,. The current ladder 10 is operated by employingthe differential current driver 14 to cause smoothly changing,complementary currents to flow from the terminals 20 and 22 through suchdifferential current driver. The current driver 14 contains a currentsource 93 and a current dividing circuit' 94, the detailed structure andoperation of which will be explained in connection with FIG. 4. However,the function of the current driver 14 may be explained as follows.

An external voltage E is supplied to the current dividing circuit 94 ofthe current driver 14 through an input terminal 95 and this voltagevaries according to the relation E XE where E is a predetermined maximumvalue of E and X varies from to 1. For example, if X varies linearlyfrom 0 to l a linear voltage ramp, such as the voltage ramp 96 of FIG. 2varying from 0 to E will be applied through the terminal 95. The currentdriver 94 will sink the current i, and as the input voltage E varieswith X, the current through the terminal 97 of the current dividingcircuit 94 of the current driver 14, and the terminal 20 of the currentladder will equal Xi and the current through the terminal 98 of thecurrent driver and the terminal 22 of the current ladder will equal (1X) i,. As X increases from 0 to I, the current flowing from the ladder10 through terminal 22 decreases linearly from i, to O and the currentfrom the ladder through terminal increases linearly from 0 to i, asindicated in FIG. 2. The sum of the currents from the ladder 10 out ofthe terminals 20, 22 is thus maintained constant but the fractionthrough each terminal varies complementarily in a smoothly changingmanner.

For discussion purposes the number of stages or node points in theladder is assumed to be equal to 5, Le, n 5. Further, at a time t= t letX= 0 so that all the current from the ladder to the differential currentdriver 14 is through terminal 22 as shown by the curve 99 of FIG. 2. Thecurrent through the terminal which causes the regulator 12 to controlthe unit sources 50, 52, 54, 56 and 58 to each deliver a current to thecurrent ladder equal to the unit current I is actually such unit currentI minus the total base current or currents of selected ones of thetransistors 72, 74, 76, 78 and as described below. This total basecurrent remains very nearly constant so that the current flow from thecurrent ladder 10 through the terminal 70 can be'employed to control theunit current sources by making the resistance value of the resistor 88of the regulator 12 greater than that of the resistor 86. The totalcurrent flowing out of the current ladder 10 through terminals 20 and 22will be (n 1) I or 4]. That is, a current of one I is supplied toterminal 22 from each of the current sources 52,54, 56 and 58 via diodes30, 34 and 38. The unit current source 50 will then supply the unitcurrent of one I required by the regulator 12 andas base current fortransistors 72, 74, 76, 78 or 80.

Under the circumstances just described there will be a voltage peak V,at the terminal 20, i.e., node 40 of the current ladder, at time t asindicated in the table 100 of FIG. 2. It is clear that the' voltage atnode 42 at this time will be higher than at any of the nodes 44, 46 or48 because of the voltage drops across the diodes 30, 34 and 38 whichconduct current to the terminal 22. The voltage at node 42 will howeverbe lower than that necessary to render diode connected transistor 62 tobecome conducting since the voltage at the node 42 will not go anyhigher than that necessary to cause the unit current source 52 to supplyone unit current I to the current ladder 10 which, with unit currentsources 54, 56 and 58, will supply the current of 41 required to flowthrough the terminal 22 at time t I The voltage at the node 40 willhowever be. sufficiently higher than voltage at node 42 to cause thediode connected transistor 60 to become conducting, the voltage at thenode 40 being determined by the voltage necessary to supply the currentrequired by the voltage regulator 12 through the terminal 70 and thebase current of transistor 70, this voltage being determined by thevoltage from the voltage source 91 and the voltage drop caused bycurrent flow through the diode connected transistor 60 and the resistor88. Thus, at time t, node 40 will be at a voltage maximum or peak asindicated in the table of FIG. 2, while each of the nodes to the rightwill be progressively more negative. Only diode connected transistor 60willconduct current to terminal 70 and the base of transistor 72, whilethe current from sources 52, 54, 56 and 58 will all flow out throughterminal 22. Conduction of transistor 72 connected to node 40 willenable current i to flow from ground potential through the outputnetwork 11 and through the collector emitter path of this transistor andthen through the current source 84 to the negative voltage supply 82.

At a later point in the cycle indicated by t, of FIG. 2, an increase ininput voltage at the terminal of the current driver 14 will cause thefactor X of the relation Xi to increase to a point where a current [/2is flowing out of terminal 20. The current out of terminal 22 will thenbe 41 [/2 or 7/2 I. In this case, the unit current I from current source50 will be evenly split; [/2 flowing through terminal 20 to the currentdriver 14 and [/2 flowing through diode connected transistor 60. Tosatisfy the regulator 12 and the base currents of transistors 72 and 74,both of which will be conducting, an additional [/2 must be supplied byone of the remaining current sources 52, 54, S6 or 58. The voltagesalong the current ladder to the right of node 40 will increase until thevoltage at node 42 will cause diode connected transistor 62 to conductand supply current [/2 from the current source 52. The voltage at bothnodes 40 and 42 will be equal and will provide the voltage peak V asindicated by the table 100 of FIG. 2. Thus diode connected transistor 62will conduct the additional current [/2 to terminal 70 required tosatisfy the regulator 12 and the base current of transistors 72 and 74.

Under the conditions just described, in which the nodes 40 and 42 are atthe same voltage, both transistors 72 and 74 will conduct equally. Thecurrent i from the output network 11 to source 84 will split evenlybetween the transistors one-half going through transistor 72 andone-half going through transistor 74. For intermediate values of theincreasing input voltage E on the voltage ramp 96 of FIG. 2 betweentimes t and t the conduction of diode connected transistor 60 andtransistor 72 will decrease and the conduction of diode connectedtransistor 62 and transistor 74 will increase, as such input voltageincreases. There is thus a smooth current transition between transistors72 and 74 which varies linearly with the input voltage E applied to theterminal 95 of the current driver 14.

As the voltage of the ramp 92 continues to increase a still greaterportion of the current from the source 50 will flow through terminal 20and the current flow from node 40 through diode connected transistor 60will decrease correspondingly. The current flow through diode connectedtransistor 62 will therefore increase, so that the total current flow toterminal 70 remains a constant as-required by regulator 12. At a timethe current flow through terminal 20 is I, the entire output of currentsource 50. Accordingly, no current will flow through diode connectedtransistor 60 and transistor 72 will have ceased to conduct current andall of the current to terminal 70 will be from current source 52 throughdiode connected transistor 62. Transistor 74 alone will have asufficiently positive base to conduct the current i from the outputnetwork to the current source 84. Node 42 only will be at the voltagepeak at time 1 V,,, as is shown in FIG. 2. In a similar manner, thevoltage peak will continue to move from left to right in response to anincrease of the voltage E applied to the terminal 95 of the currentdriver 14. Thus, the transistors 72, 74, 76, 78 and 80 will besequentially rendered conducting when the voltage peak arrives at therespective nodes as indicated in FIG. 2. A cycle of operation iscompleted when the last transistor 80 has been rendered conducting andthe voltage E is then returned to zero.

While the number of stages or nodes in the above discussion was assumedto be 5, it will be readily apparent that a very much greater number ofstages will ordinarily be employed. The limiting factor is the reversebreakdown voltages of the transistors connected to the nodes atterminals 20 and 22 at the ends of the current ladder 10. As the numberof nodes are increased, the voltage at the terminal 20 or 22 of thecurrent ladder remote from the voltage peak at the beginning or endportions of the cycle will exceed the reverse breakdown voltage of thebase-emitter junctions of the transistors adjacent such terminal. Thevoltage limiter 16 shown in FIG. 1 may be employed to effectivelyshorten the current ladder by dividing it into sections during a cycleof operation.

The voltage limiter 16 comprises a pair of NPN tran sistors 101 and 102.The base leads 104 and 106 of such transistors are connected to anegative voltage reference determined by the voltage source 107 havingits negative terminal connected to the negative terminal of the voltagesource 91 and its positive terminal connected to ground. The emitterleads 108 and 110 are connected to the terminals 20 and 22,respectively, of the current ladder 10. The collector leads 112 and 114are tied together and connected to the node 44 of the current ladder 10at or adjacent the midpoint of the ladder.

The voltage limiter l6 diverts current from a central node of thecurrent ladder 10 before the voltage at one of the terminals 20 or 22becomes sufficiently negative to cause transistor breakdown. Thustransistor 10] will have its emitter driven negative with respect to itsbase to cause such transistor to conduct current from node 44 of thecurrent ladder 10 which would otherwise flow through the diodes, such asdiodes 34 and 38, connected between node 44 and the terminal 20.Transistor 102 will similarly become conducting near the beginning ofsuch cycle to conduct current from the node 44. In either case, the node44 of the current ladder connected to the collector of the transistors101 and 102 becomes an effective end of the current ladder 10. A voltagepeak or near peak will occur at the same node of the current ladder l0irrespective of whether the voltage limiter l6 diverts current from theladder. Thus the voltage limiter will not affect the ladder operationother than to enable its length to be made greater. For example, at thebeginning of a cycle, the voltage peak will be at node 40. All nodes tothe right in FIG. 1 will be at a lower potential due to the voltage dropacross diode 26. Thus it makes nodifference whether the current flowingthrough nodes to the right of node is withdrawn from the ladder atterminal 22 or node 44.

The output resistor network 1 1 for the current ladder 10 is aprogrammable network including a plurality of current dividers made upof resistor pairs 140, 142; 144, 146; 148, I; 152, 154 and 156, 158. Theamplifier 160 and associated feedback resistor 161 is also a part ofsuch network.

If one only of the transistors 72, 74, 76, 78 and 80, for example thetransistor 72 at time t,,, is conducting, the current i flows throughthe current divider resistor pair connected thereto, for example theresistor pair and 142. Thus the current source 84 will cause the emitterof the transistor 72 to go sufficiently negative to cause the collectorcurrent of such transistor to be the current i The voltage V at theoutput 162 of the amplifier will then be directly proportional to thecurrent i and the resistance of the resistor 142. Thus it is easilyshown that V i (R R /R R where the R indicates the resistance of theresistor having the various subscripts. The resistance of R will have aconstant value and while not essential, the sum R R can also be aconstant. In this case V Ki R where K is a constant. Since i is alsoheld constant V will be directly proportional to the resistance of 142If two of the transistors 72, 74, 76, 78 and 80, such as the transistors72 and 74 are conducting as discussed above, so that the constantcurrent i divides between the two resistor pairs 140, 142 and 144, 146,the output voltage V of the amplifier 161 will be intermediate in valuebetween such output voltage when all of the current i flows through thetransistor 72 and resistor pair 140, 142 and such output voltage whenall of the currenti flows through transistor 74 and resistor pair 144,146. For example when the peak voltage along the voltage ladder causesequal current /2 to flow through the transistors 72 and 74 and currentsource 84, the voltage output at the terminal 162 of the amplifier 160will be V K (R R )/2. This is the average of the two output voltagesobtained when all of the current i flows through transistor 72 and thenall of the current flows through transistor 74. There is thus asubstantially linear transition from the output voltage when the voltagepeak V along the current ladder 10 is at one node and the output voltagewhen this peak is at the next adjacent node and such peak is moved alongthe ladder from one of such nodes to the other by varying the voltageinput to the terminal 95 of the current driver 14.

In the same manner, the output voltage at the output terminal 162 of theamplifier 16 will vary according to preset current divider resistorratios as the corresponding transistors 72, 74, 76, 78 and 80 aresequentially caused to conduct. The output voltage is thus aprogrammable function of the input voltage at n evenly spaced values. Ifthe value of the current i is properly regulated so as to be heldconstant, the values of output voltage between the n points will be asubstantially linear interpolation of the output voltages at the nearestpoints. It is desirable to provide a maximum number of programmablepoints, n, in order that a desired waveform may be more closelyapproximated by the function generator. The voltage limiter 16 describedpreviously, enables a larger number of programmable points to beemployed without danger of damage to the transistors in the currentladder.

The effects of temperature upon the transistors 72, 74, 76, 78 and 80 ofthe current ladder 10 must be compensated for if a selected outputwaveform is to be reproducible at any time. This is accomplished byaccurately temperature compensating the current source 84 in a mannerwhich holds the total collector current of such transistors very closeto a constant value under changing temperature conditions. Such totalcollector current is the current i; from the current divider resistornetwork I]. The total current through the current source 84, however,also includes the base current of one or two such transistors which areconducting as discussed above with respect to the current regulator 12.The current flowing through the current source 84 is therefor equal to i(1 H3) where ,8 is the current amplification of any one of thetransistors 72, 74, 76, 78 or 80. The circuit of FIG. 3 is a compensatedcurrent sourcewhich may be employed as the current source 84 of FIG. 1to temperature stabilize the output current of such transistors eventhough the current gains B of such transistors all change withtemperature. The circuit is similar to that disclosed in U. S. Pat. No.

3,588,672 except that it employs an additional transistor 205.

The compensated current source of FIG. 3 includes the NPN transistors200, 202, 204 and 205. Each tran sistor has an identical current gain Bwhich is also the same as the current gain of each of the transistors72, 74, 76, 78 and 80. The bases of transistors 202, 204 and 205 arecoupled together and to the emitter of transistor 200 so that the basecurrents of transistors 202, 204 and 205 are part of the emitter currentof transistor 200. The emitters of transistors 202, 204 and 205 arelikewise coupled together and connected to a negative voltage supply.Transistor 202 also has its collector coupled to the emitter oftransistor 200 and thus functions as a diode. The collector oftransistor 200 is connected through terminal 83 to the emitters of thecurrent ladder transistors 72, 74, 76, 78 and 80 for receiving thecurrent i (1 l/B) from such transistors. The collector of transistor 205is coupled to ground through a resistor 205'. A constant current i issupplied to terminal 206 from a controlled current source 207. Most ofthis current flows through the collector emitter path of transistor 204while a small portion supplies the base current of transistor 200.

Assume the base current of the transistor 200 can be very nearly equalto i /[3 so that the collector current of the transistor 204 is equal toi (1 H3). The bases of the transistors 202 and 204 are connectedtogether and the same is true of the emitters of these transistors. Alsotransistor 200 is an emitter follower so that the collector voltage oftransistor 202 is very nearly the same as the collector voltage oftransistor 204. Under these conditions the collector current oftransistor 202 is very close to being the same as the collector currentof transistor 204 and is also i (l U3).

The transistor 205 also has its base and emitter connected to the baseand emitter respectively of the transistor 204, and the collectorvoltage of the transistor 205 is very close to the collector potentialof transistor 204. The base currents of all three transistors 202, 204and 205 are therefor all equal to each other and to i (l U3) (U3) whichis very nearly equal to i /B. This is true since 1),,(1 U3) (U3) equalsi l/B l/B) and l/B is a very small quantity and can be neglected.

The base currents of all three transistors 202, 204 and 205 are part ofthe emitter current of transistor 200. The transistor 205 is employed inthe circuit of FIG. 3 for the purpose of adding a third base current tothe base currents of the two transistors 202 and 204 so that the emittercurrent of the transistor 200 is equal to i,,,,(l I/B) 3i,..,/B i (l2/B). The collector current of transistor 200 equals its emitter currentless i /B so that the collector current of transistor 200 is i (1 H3), imust equal i, and the base current of transistor 200 is very nearlyequal to i /B as originally assumed.

The amount of current i flowing from the current divider resistancenetwork 11 through the transistors is therefore completely controlled bythe constant current i so long as the transistors 72, 74, 76, 78 and 80all have the same current gain as the transistors of the circuit of FIG.3.

The current source 84 is itself temperature compensated so that therelation between the reference current i,,,, and the collector currentof transistor 200 remains very close to being constant except for theIII; term.

This is the temperature compensation of US. Pat. No. 3,588,672 and canbe briefly explained as follows.

If the collector current of the transistor 200 tends to increase becauseof a temperature increase, the collector current of transistor 204increases by a substantially similar amount also because of suchtemperature increase. Since the reference current i is constant, thebase current of transistor 200 is decreased to decrease the collectorcurrent of transistor 200 to hold such collector current substantiallyconstant. Any change in the base currents of transistors 72, 74, 76, 78and 80 is compensated by a similar change in the base currents oftransistors 202, 204 and 205 so that the current i flowing from thecurrent divider resistor network 11 through the transistors 72, 74, 76,78 and 80 remains substantially equal to i The accuracy of the controlof the constancy of the current i is such that it is feasible to employturn precision potentiometers to accurately set the values of theresistor pairs of the current divider resistor network 11.

FIG. 4 shows the preferred embodiment of the differential current driver14. The voltage input circuit for the current driver includes a resistor250 connected in series between the voltage input terminal 95 and thenegative input of a differential amplifier 252, the positive input ofsuch amplifier being connected to ground. The output of the amplifier250 is connected to the gate of an P channel field effect transistor 254which has its source connected back to the negative input of theamplifier and its drain connected through the collector-emitter path ofa diode connected NPN transistor 256, a diode 258 and thecollector-emitter path of an NPN transistor 260 to a negative voltagesource which may be the voltage source 82 of FIG. 3.

The connection of the source of the field effect transistor 254 to thenegative input of the amplifier provides a source follower negativefeedback to such negative input to hold such negative input at groundpotential as the voltage E at the input terminal 95 is increased in apositive direction. The current through the resistor 250 is E/R where Ris the resistance of the resistor 250 and this is also the drain currentof the transistor 254, which is the input current i of the voltagedivider circuit 14. This input current i,,, is a linear function of theinput voltage E.

As stated previously the voltage E is varied from zero to E as thefactor X varies from 0 to 1 and the current i, likewise varies from O toi as X varies from 0 to 1. The current i is the input current which willcause the current through the terminal of the current ladder l0 and theterminal 97 of the current driver 14 to change from zero to i accordingto the relation Xi and the current through the terminal 22 of thecurrent ladder I0 and the terminal 98 of the current driver 14 to changefrom i to zero according to the relation i X)|- Most of any current ifrom the field effect transistor 254, when the input voltage E isgreater than zero, flows through the diode connected NPN transistor 256and the remainder supplies base current of a current divider NPNtransistor 262 which has its collector connected to the terminal 97 andits emitter connected to the emitterv of another current divider NPNtransistor 264 having its collector connected to the terminal 98. Thebase of the transistor 264 is connected to a negative voltage source 266which is also connected to base and collector of a diode connected NPNtransistor 268 which has its emitter connected to the emitter of thediode connected NPN transistor 256. As explained below, when the currenti,-, from the field effect transistor 254 is less than i current flowsfrom the voltage source 266 through the diode connected transistor 268and then through the diode 258 and transistor 260 to the voltage source82. The voltage source 266 also supplies base current to the transistor264.

The emitter of the transistors 262 and 264 are connected to thecollector of an NPN transistor 270 which has its emitter connected tothe collector and base of a diode connected NPN transistor 272. The baseof the transistor 272 is also connected to the base of the transistor260 and to the base of another NPN transistor 274 which has itscollector connected to a controlled current source 276 which furnishes areference current i most of which flows through the collector-emitterpath of transistor 274. The base of the transistor 270 is also connectedto the current source 27 6 so that the remainder of i is the basecurrent of transistor 270.

The emitters of transistors 272 and 274 are connected to the emitter oftransistor 260 and to the voltage source 261. All of the NPN transistorsof FIG. 4 have the same current gain B. The transistors 260, 27 0, 272and 274 are connected in the circuit of FIG. 4. in exactly the samemanner as the transistors 205, 200, 202 and 204 of the temperaturecompensated current source 84 of FIG. 3 and operate in the same manner.

If the transistors 260, 272 and 274 are identical, the collectorcurrents of each of these transistors is equal to i (l l/B) so that theemitter current of transistor 270 is equal to i (l 2/B) and thecollector current i which is the total emitter current of the currentdivider transistors, is equal to i (l H3). The total collector current iof the transistors 262 and 264 is then equal to i since it is thecurrent i, less the total base currents of such transistors. Thiscurrent i is equal to (n l) I where I is the unit current of theregulated current sources 50, 52, 54, 56 and 58 of the current ladder 10 and n is the number of nodes of such ladder.

Since the collector current i, of the transistor 260 is equal to i (ll/fi), the maximum current from the field effect transistor is also i asit also contains the total base currents of the transistors 262 and 264.The maximum voltage E which is applied to the input terminal of thecurrent driver 14 is then i times R When the input voltage E applied tothe input terminal 95 of the current driver 14 at time t, is zero, theinput current i, is also zero. Under these conditions all of thecollector current for the transistor 260 plus the base current of thecurrent divider resistor transistor 264 is supplied from the voltagesource 266 so that the current from such source at such time is alsoi,,,,. The collector current of this transistor is i i since X equalszero at time t,,. The circuit of FIG. 4 containing the transistors 260,270, 272 and 274 has the same temperature compensating properties as thecircuit of FIG. 3.

The total input current from the field effect transistor 254 and thevoltage source 266 is held at the constant value i and as the current ifrom the field effect transistor 254 is increased, the current from thevoltage source 266 complementarily decreases.

The voltage drops through the diode connected transistors 256 and 268vary exponentially with the currents therethrough to cause the basecurrents of the transistors to vary with the currents through the diodeconnected transistors 256 and 268, respectively, so that the collectorcurrents of the transistor 262 varies directly with X, i.e., with theinput current i, from the field effect transistor 254, and thus directlywith the input voltage E applied to the input terminal 95. The result isan accurate transition of the current i, (n l I from the terminal 20 ofthe current ladder to the terminal 22 of such ladder and this transitionis a linear function of the input voltage E from the input terminal 95of the current driver 14 and of the input current i from the fieldeffect transistor 254. If E varies linearly with time as indicated bythe voltage ramp 96 of FIG. 2, the current through the terminal varieslinearly from zero to (n 1) I, as indicated by the current ramp 99 andthe current through the terminal varies linearly from (n l) I to zero asindicated by the current ramp 278 so that the total current from theterminals 20 and 22 is always (n l)! i In the above discussion of thecircuit of FIG. 4, it was assumed that all of the transistors of suchcircuit are identical. However a current i which is a factor y times thereference current can be controlled by a given reference current i Thiscan be done by scaling up the transistors 272 as well as transistors262, 264 and 270 so that their base-emitter and base-collector junctionshave y times the areas of the similar junctions of the other transistorsin such circuit. By so doing i yi where y can have a value of 4, forexample, or even a larger value. It can be shown that this does noteffect the. accuracy of the control of i by the reference current i andthat the maximum value i of the input current from the field effecttransistor 254 remains equal to i,,,,.

All of the circuits described above cooperate to provide an extremelyaccurate function generator which can be programmed to produce anydesired function. The current regulator 12 insures that the totalcurrent supplied to the current ladder by the unit current sources 50,52,54, 56 and 58 is exactly the complementary currents required by thedifferential current driver 14 plus the base currents of the outputtransistors 72, 74,76, 78 and 80 and the current required by theregulator 12 itself. The regulator 12 makes the function generatorstable, accurate and very independent of any common mode current fromthe differential current driver 14.

It will be apparent that current ladder 10 may. have other types ofseries impedances, such as resistors, instead of the reversely connecteddiodes shown in FIG. 1 and that such ladder with its associatedregulator 12 and output transistors can be employed with any othersuitable current source for providing a constant output current fromsuch transistors and with any other suitable current driver forregulating and complementarily varying the currents flowing from theterminals 20 and 22. It should be noted that these currents can bevaried other than linearly according to any desired input functioneither by the current driver circuit shown in FIG. 4, or other suitablecurrent driver. Also the outputs from the collectors of such transistorsof such current ladder can be employed in output circuits other than thecurrent divider resistor network 11 shown in FIG.

The circuit of the controlled and compensated current source of FIG. 3can also be employed wherever an accurate current source with a basecurrent component is required and similarly the current driver circuitof FIG. 4 wherever it is desired to complementarily vary two currents.It is to be noted that the current flowing through the terminal 83 ofthe circuit of FIG. 3 can be made to have any desired value with respectto the current i from the current source 207 by scal ing up the junctionareas of the transistors 202 and 200 in the same manner described withrespect to the transistors 272, 270, 262 and 264 of FIG. 4.

I claim: 1. A programmable function generator comprising: current laddermeans including a pair of terminals, a plurality of impedances in serieshaving nodcs therebetween and a plurality of matched current sourcemeans each connected to one only of said nodes for delivering current tosaid ladder; current driver means for drawing complementary currentsfrom said ladder means through said pair of terminals adding to aconstant value and varying said currents for causing a voltage peak tomove along said ladder means; a plurality of output means, eachincluding programmable impedance means for producing an outputdetermined by current flow through said impedance means;

a plurality of transistor means each connected to one only of said nodesand one .only of said output means and drawing current from said ladderresponsive to said voltage peak for enabling current flow from selectedones of said output means;

and regulating means drawing current from said ladder means andresponsive thereto for regulating said current source means to maintainthe total current delivered to said ladder means equal to the currentrequired by the means drawing current from said ladder means.

2. The generator of claim 1 wherein said plurality of output means areconnected to means for converting said output to a voltage.

3. The generator of claim 1 wherein said current driver means isresponsive to an externally applied sig nal to vary said complementarycurrents as a linear function of said external signal.

4. The generator of claim 1 wherein said current driver means includestemperature compensating means for maintaining the total current fromsaid ladder means at said constant value.

5. The generator of claim 1 wherein said generator includes anadditional current source means for sinking said current from saidoutput means and said additional current source means has feedback meansfor maintaining said current from said plurality of output meansconstant.

6. The function generator of claim I further including voltage limitermeans for diverting current from a central node of said ladder meanswhen the voltage from either of said pair ofterminals to said centralnode approaches the reverse breakdown voltage of any of said transistormeans.

7. The function generator of claim 5 wherein said feedback meansincludes means for compensating the current flow from said plurality ofoutput means against variations due to temperature;

and means for maintaining the current flow from said output means tosaid transistor means constant so that during transitions from a firstoutput means to a second output means said output will be a linearinterpolation between the values of the output from said first andsecond output means 8. The generator of claim 4 wherein said currentdriver means further includes means for causing the complementary flowfrom said terminals of said ladder means to shift linearly between saidterminals.

9. A function generator comprising:

a series impedance means having a .=plurality of impedance meansconnected in series between a pair of terminals and defining circuitnode points at said terminals and between said impedance means;

a plurality of matched current source means for providing equal currentsand each connected to one only of said circuit node points to supplysaid equal currents to said ladder;

current driver means for providing flow of complementary current fromsaid ladder through said pair of terminals in response to an externallyapplied signal to provide a voltage peak in said ladder at a circuitnode or at two of said nodes of said ladder, and for controlling saidcomplementary currents to determine the position of said voltage peakalong said ladder; plurality of unidirectional conducting means eachconnected between one only of said nodes and a third terminal of saidladder to provide for current flow through said third terminal from saidcircuit node or nodes which are at said voltage peak; regulating meansresponsive to current flow through said third terminal for controllingsaid matched current source means to maintain the current from 10. Thefunction generator according to claim 9 in which said impedance meanseach comprise a pair of diodes which are connected in parallel and forconduction in opposite directions.

11. The function generator according to claim 9 in which said outputcontrol means comprise transistors having a control element connected tothe corresponding one of said node points and drawing current from saidladder responsive to said voltage at said one node.

12. The function generator according to claim 11 which also includesvoltage limiter means for diverting current from a central node of saidladder when the voltage from either of said pair of terminals to saidcentral node approaches the reverse breakdown voltage of any of saidtransistors.

13. The function generator according to claim ll in which saidregulating means comprises:

a pair of matched impedances;

an additional matched current source means;

and differential amplifier means responsive to the difference betweenvoltages produced by current flow from said third terminal through oneof said matched impedances and current flow from said additional currentsource means through the other of said matched impedances for regulatingthe current output of each of said plurality of matched current sourcemeans to maintain the total current from said ladder through said thirdterminal and said transistors equal to the current from one of saidmatched current source means.

14. The function generator according to claim 9 including means tosupply an input control current derived from said externally appliedsignal to said current driver, said current driver having meansresponsive to said input control current to vary said complementarycurrents as a linear function of said input control current.

1. A programmable function generator comprising: current ladder meansincluding a pair of terminals, a plurality of impedances in serieshaving nodes therebetween and a plurality of matched current sourcemeans each connected to one only of said nodes for delivering current tosaid ladder; current driver means for drawing complementary currentsfrom said ladder means through said pair of terminals adding to aconstant value and varying said currents for causing a voltage peak tomove along said ladder means; a plurality of output means, eachincluding programmable impedance means for producing an outputdetermined by current flow through said impedance means; a plurality oftransistor means each connected to one only of said nodes and one onlyof said output means and drawing current from said ladder responsive tosaid voltage peak for enabling current flow from selected ones of saidoutput means; and regulating means drawing current from said laddermeans and responsive thereto for regulating said current source means tomaintain the total current delivered to said ladder means equal to thecurrent required By the means drawing current from said ladder means. 2.The generator of claim 1 wherein said plurality of output means areconnected to means for converting said output to a voltage.
 3. Thegenerator of claim 1 wherein said current driver means is responsive toan externally applied signal to vary said complementary currents as alinear function of said external signal.
 4. The generator of claim 1wherein said current driver means includes temperature compensatingmeans for maintaining the total current from said ladder means at saidconstant value.
 5. The generator of claim 1 wherein said generatorincludes an additional current source means for sinking said currentfrom said output means and said additional current source means hasfeedback means for maintaining said current from said plurality ofoutput means constant.
 6. The function generator of claim 1 furtherincluding voltage limiter means for diverting current from a centralnode of said ladder means when the voltage from either of said pair ofterminals to said central node approaches the reverse breakdown voltageof any of said transistor means.
 7. The function generator of claim 5wherein said feedback means includes means for compensating the currentflow from said plurality of output means against variations due totemperature; and means for maintaining the current flow from said outputmeans to said transistor means constant so that during transitions froma first output means to a second output means said output will be alinear interpolation between the values of the output from said firstand second output means.
 8. The generator of claim 4 wherein saidcurrent driver means further includes means for causing thecomplementary flow from said terminals of said ladder means to shiftlinearly between said terminals.
 9. A function generator comprising: aseries impedance means having a plurality of impedance means connectedin series between a pair of terminals and defining circuit node pointsat said terminals and between said impedance means; a plurality ofmatched current source means for providing equal currents and eachconnected to one only of said circuit node points to supply said equalcurrents to said ladder; current driver means for providing flow ofcomplementary current from said ladder through said pair of terminals inresponse to an externally applied signal to provide a voltage peak insaid ladder at a circuit node or at two of said nodes of said ladder,and for controlling said complementary currents to determine theposition of said voltage peak along said ladder; a plurality ofunidirectional conducting means each connected between one only of saidnodes and a third terminal of said ladder to provide for current flowthrough said third terminal from said circuit node or nodes which are atsaid voltage peak; regulating means responsive to current flow throughsaid third terminal for controlling said matched current source means tomaintain the current from said node or nodes which are at said voltagepeak to said third terminal equal to the current from one only of saidmatched current source means; and a plurality of output control meanseach connected to one only of said node points and responsive to thevoltage at the corresponding one of said node points.
 10. The functiongenerator according to claim 9 in which said impedance means eachcomprise a pair of diodes which are connected in parallel and forconduction in opposite directions.
 11. The function generator accordingto claim 9 in which said output control means comprise transistorshaving a control element connected to the corresponding one of said nodepoints and drawing current from said ladder responsive to said voltageat said one node.
 12. The function generator according to claim 11 whichalso includes voltage limiter means for diverting current from a centralnode of said ladder when the voltage from either of said pair ofterminals to said central node approaches The reverse breakdown voltageof any of said transistors.
 13. The function generator according toclaim 11 in which said regulating means comprises: a pair of matchedimpedances; an additional matched current source means; and differentialamplifier means responsive to the difference between voltages producedby current flow from said third terminal through one of said matchedimpedances and current flow from said additional current source meansthrough the other of said matched impedances for regulating the currentoutput of each of said plurality of matched current source means tomaintain the total current from said ladder through said third terminaland said transistors equal to the current from one of said matchedcurrent source means.
 14. The function generator according to claim 9including means to supply an input control current derived from saidexternally applied signal to said current driver, said current driverhaving means responsive to said input control current to vary saidcomplementary currents as a linear function of said input controlcurrent.